1. Field of the Invention
The invention relates generally to a target source for film deposition, and more particularly to a target source for depositioning films of a complex chemical composition using ion beam sputtering techniques.
2. Description of the Prior Art
Sputter deposition processes using ion beam sources are found in a variety of applications. The advantages of using ion beam sputter deposition are isolation of the substrate from energetic electron bombardment and independent control over the ion energy and current density striking the target. The basic techniques of ion beam sputter deposition are well known in the art. See, for example, H. R. Kaufman, J. J. Cuomo and J. M. E. Harper, Technology and Applications of Broad-Beam Ion Sources Used in Sputtering, J. Vac. Sci. Technol., 21, (3), Sept./Oct. 1982. The process in general involves the use of a low energy plasma source in a magnetic field which develops a neutralized ion beam, with the ion energy, direction, and current density all independently controllable and all independent of the type of target used. The plasma is produced by direct current electron bombardment of a low pressure gas, generally inert, but which may have a small percentage of reactive gases.
Generally, any target material which can be physically emplaced in a vacuum chamber can be sputtered by this process. This includes powders and composite targets, as well as elemental materials. When an insulating target is used, a neutralizer is required to avoid positively charging this insulator surface. Such charging may cause sparking across the target surface, voltage breakdown through the target, or deflection of the ion beam. A neutralizer will supply electrons and avoids surface charging.
In depositing a film of a given chemical composition, where the target is relied on as the primary source of material, it is accepted practice to prepare the target from a given composition. The target source is typically 4 to 8 inches in diameter, depending on the size of the enclosure, and nominally 0.1 to 0.3 inch thick. It is prepared by mixing the desired proportions of chemical reagents and pressing the powders together at elevated or room temperature. However, where multi-component targets are utilized, such effects of ion bombardment as preferential sputtering, formation of an altered surface layer, enhanced diffusion, surface texturing, compound formation and phase transformation may strongly influence the resultant film. Further, the film composition is not necessarily the same as the target composition due to differences in ion sputtering coefficients and film sticking coefficients.
The prior art has relied on making several iterations of target materials to achieve film depositions of desired chemical composition. This is an expensive and time consuming empirical procedure and has not been wholly satisfactory. An alternative approach to depositing multi-component films used in the prior art is to allow the beam to strike a segmented target. As reported by Harper, et al (op.cit.) cermets of Ag/SiO.sub.2 were deposited over a range of compositions of 55-92-wt. % Ag using two adjacent targets of Ag and SiO.sub.2 . (Originally reported by P. Reinhardt, et al, Thin Solid Films 51, 99 (1978). As reported therein, the film composition was controlled by lateral positioning of the targets in the beam to sputter varying fractions of the two targets, thereby allowing the film resistivity to be varied over a range of six orders of magnitude. Thus, target texture is seen to affect film composition.
It is clear, therefore, that elemental sputter yields may not be used to predict sputter yields in alloys or components and that the sputter yield ratios in alloys may be strongly composition dependent. Therefore, in thin film deposition using ion sputtering, the final film composition is difficult to predict.
The present invention provides an improved target source that allows modification of the target composition in precisely controlled increments without the necessity for fabricating an entirely new target.